1. Field of the Invention
The present invention relates to a charging member, a process cartridge including the charging member, and an image forming apparatus including the charging member. More particularly, the present invention relates to a charging member that is disposed opposite to an image carrying member in a close but non-contact manner for forming an image with an electrophotographic image forming method, a process cartridge including such charging member, and an image forming apparatus including such charging member. Such an image forming apparatus corresponds to a copier, laser beam printer, facsimile machine, and so forth that uses an electrophotographic image forming method.
2. Discussion of the Related Art
Related art electrophotographic image forming systems such as copiers, laser beam printers, facsimile machines, and so forth generally include a conductive member, for example, a charging member for charging an image carrying member or a photoconductor, and/or a transfer member for transferring a toner image formed on an image carrying member.
A well known technique for charging an image carrying member with a charging roller as a charging member includes a non-contact charging method to keep a desired performance ability of the charging roller as it ages.
In the above-described technique, a charging roller and a photoconductor serving as an image carrying member are disposed opposite to each other. The closest distance or gap between the charging member and the photoconductor is in a range from approximately 50 μm to approximately 200 μm. With the above-described configuration, a given amount of voltage is applied to the charging roller so as to charge the photoconductor.
With the non-contact charging method, the charging member and the photoconductor are not held in contact with each other. Therefore, various problems arising from using a contact charging method can be prevented. Specifically, adhesion of material of a charging roller to a photoconductor, permanent deformation of a photoconductor caused while stopping for a long period of time, and so on may not be caused.
In addition, another problem such as deterioration in charging ability due to adhesion of toner on a photoconductor to a charging roller may be reduced more with the non-contact charging method because less toner may adhere to the charging roller.
However, even with the above-described advantages, it is difficult to use the non-contact charging method in an electrophotographic image forming apparatus due to the following reasons:
1. Formation of a uniform gap between a charging member and a photoconductor is difficult; and
2. Gap variation between a charging member and a photoconductor may cause charging nonuniformity.
For the difficulty in forming a uniform gap of closest distance between a charging member and a photoconductor, a charging member may need to charge a photoconductor opposite to a given close gap therebetween so as to not produce a defective image due to the charging nonuniformity. To avoid producing such a defective image, the deviation in distance between the charging member and the photoconductor needs to be, ideally, approximately 20 μm at the closest non-contact part.
In a related art image forming apparatus including the above-described technique, spacer rings that serve as a gap retaining member are disposed at both ends of the charging roller so that the gap formed between the charging roller and the photoconductor can be constantly retained.
However, the above-described technique has not shown a detailed method of precisely setting the gap. In addition, the deviation of dimensional accuracy of the charging roller and the spacer rings can vary the distance of the gap.
A related art image forming apparatus employing a different well known technique includes a charging roller having an elastic rubber material and a gap retaining member in a form of a tape having a given thickness. This structure has eliminated the above-described disadvantages. However, the size of the elastic rubber material included for the charging member can easily vary with time due to aging, and therefore, the charging roller and the photoconductor cannot form a constant gap for a long period of time of use. In addition, the above-described structure has caused different disadvantages, for example, abrasion of the tape-type gap retaining member, toner falling and sticking between the charging roller and the tape-type gap retaining member. Due to these disadvantages, the gap between the charging member and the photoconductor cannot be maintained.
To eliminate these disadvantages, another technique has been provided to include gap retaining members mounted at both ends of a charging roller, as shown in FIG. 1.
In FIG. 1, a related art charging roller 10 includes a conductive supporting member 1, an electrical resistance control layer 2, and gap retaining members 3. Specifically, the gap retaining members 3 are mounted at both ends in a longitudinal direction of the electrical resistance control layer 2 of the charging roller 10. The gap retaining members 3 are held in contact with the electrical resistance controller layer 2 on both end surfaces in a longitudinal direction of the electrical resistance controller layer 2 and the conductive supporting member 1 at both ends in a longitudinal direction of the conductive supporting member 1. With the structure as shown in FIG. 1, the performance ability and reliability of the gap retaining member for a long-time use has been enhanced when compared with the tape-type gap retaining member.
Further, in a related art image forming apparatus with a further different known technique, a gap retaining member and an electrical resistance control layer are processed with a removal process at a concurrently same time so as to precisely control the gap formed therebetween. However, when the gap retaining member and the electrical resistance control layer are formed by different materials, their respective coefficients of water absorption may be different. Thus, when the environment around the related art image forming apparatus changes, the gap retaining member and the electrical resistance control layer may change in size by different amounts which may result in a change of the amount of the gap.
In addition, a gap retaining member and an electrical resistance control layer are formed with different materials having different toner sticking tendencies. The electrical resistance control layer in the above-described well-known technique includes an ion conductive layer as a resistance control agent that has a high water absorption rate. Therefore, under an environment with high temperature and high humidity, such an electrical resistance control layer absorbs humidity so that the electrical resistance control layer may swell or expand to change its size.
It is preferable that a gap retaining member is nonconductive and includes olefin material to reduce or prevent (if possible) toner sticking. With the above-described material, the gap retaining member can have a lower water absorption compared with the material of the electrical resistance control layer, and may cause a smaller size change in an environment with high temperature and high humidity. Therefore, a gap precisely formed may vary due to the environmental changes.
The gap retaining member is engaged with the charging roller by covering and capping the end portion of the charging roller. The preferable gap between the gap retaining member and the surface of the photoconductor is relatively small, e.g., in a range from approximately 20 μm to approximately 100 μm. Therefore, the gap retaining member may generally be thin, which cannot provide a volume that can maintain a rigidity thereof. In such case, a reinforcement part can be provided at an end portion of the charging member to easily reinforce the rigidity. However, if an inner portion of the electrical resistance control layer of the charging roller swells or expands as described above with time due to the process of aging, the abutting part with respect to the surface of the photoconductor may change or move up while the size of the reinforcement part does not change, which results in a disadvantage of changing the distance of the gap.